Residual stress improving method for pipe

ABSTRACT

It is an object to provide a residual stress improving method for a pipe by imparting larger compressive residual stress on the pipe to sufficiently reduce tensile residual stress in order to prevent the stress corrosion cracking. With respect to a stress improving region where the residual stress of a pipe is to be improved, a load in the axial direction of the pipe is made such stress making axial strain of the outer surface of the pipe 0% or above and being yield stress of the pipe or below, and internal pressure of the pipe is raised. The pipe is plastically deformed and is expanded in the radial direction by the internal pressure. After the internal pressure is raised to the degree the pipe is plastically deformed, the internal pressure and the axial load are removed, and thereby compressive residual stress is imparted to a welding section and a heat affected zone which are the stress improving region of the inner surface of the pipe.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent application serial No. 2009-294016 filed on Dec. 25, 2009, the content of which is hereby incorporated by reference into this application

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a residual stress improving method for a pipe, and relates specifically to a residual stress improving method for a pipe suitable in improving the residual stress in the vicinity of a welding section of a pipe.

2. Description of the Related Arts

With respect to a pipe used for a power plant, the residual stress occurs in the vicinity of the welding section when welding is performed. Also, a stainless steel pipe or a nickel alloy pipe is commonly used for a pipe for high temperature water considering corrosion resistance. With respect to a pipe using stainless steel, nickel alloy and the like, when it is exposed to high temperature pure water for a long time while being applied with tensile residual stress at the welding section of the pipe, stress corrosion cracking may possibly occur. Therefore, it is preferable to improve the residual stress by reducing the tensile residual stress generated by welding or by changing it to the compressive residual stress.

As a method for improving the residual stress of the welding section of a pipe, methods utilizing ice plugs formed inside the pipe have been proposed in preceding technical documents.

In a residual stress improving method for a pipe described in Japanese Unexamined Patent Application Publication No. 2006-334596, refrigerant containers for forming ice plugs are attached to the upstream side and downstream side of a welding section of a pipe respectively, and refrigerant containers for pipe expansion are attached between the respective refrigerant containers for forming ice plugs and the welding section respectively.

First, the ice plugs are formed in the pipe at two locations on the upstream side and downstream side of the welding section by respective refrigerant containers for forming ice plugs, and then the vicinity of the welding section of the pipe is pressingly expanded outward utilizing volume expansion of the ice formed by freezing the water existing between the ice plugs by respective refrigerant containers for pipe expansion. Thus, the vicinity of the welding section of the pipe is plastically deformed, compressive residual stress is imparted to the inner surface in the vicinity of the welding section, and the tensile residual stress is reduced or is changed to compressive residual stress.

In a residual stress improving method for a pipe described in Japanese Unexamined Patent Application Publication No. 2008-238190, excess weld metal of the welding section or the outer surface of a pipe is machined, and the thickness in the vicinity of the welding section of the pipe is made thinner than the thickness of the other portions. When the thickness in the vicinity of the welding section of the pipe is made thinner, the deformation amount in the vicinity of the welding section in expanding the pipe by internal pressure of the pipe becomes larger than the deformation amount of a part other than the vicinity of the welding section.

By expanding the pipe to the degree that plastic deformation is caused, tensile residual stress applied to the inner surface of the pipe in the vicinity of the welding section is reduced or is changed to compressive residual stress.

In a residual stress improving method for a small bore pipe described in Japanese Unexamined Patent Application Publication No. 2009-50906, in a state an axial tensile load is imparted to the vicinity of the welding section of a pipe, temperature difference is imparted between the inner and outer surfaces of the pipe, the internal pressure in the vicinity of the welding section is raised to expand the pipe, and compressive residual stress is imparted to the inner surface of the pipe.

In the residual stress improving method for a pipe described in Japanese Unexamined Patent Application Publication No. 2006-334596, the residual stress is improved by stress difference between the inner surface and outer surface in plastic deformation of the pipe. Therefore, according to the diameter and thickness of the pipe, the residual stress of the pipe may not be sufficiently improved to the degree stress corrosion cracking is prevented.

In the residual stress improving method for a pipe described in Japanese Unexamined Patent Application Publication No. 2008-238190, the strength may become lower than the required strength of the pipe when the excess weld metal or the outer surface of the pipe is machined in order to prevent the stress corrosion cracking.

Also, in a residual stress improving method for a small bore pipe described in Japanese Unexamined Patent Application Publication No. 2009-50906, compressive residual force is imparted by generating plastic strain at the inner surface by the tensile stress overlapped by the axial tensile load. In this case, according to the magnitude of the tensile load applied, the residual stress of the pipe may not be sufficiently improved to the degree the stress corrosion cracking is prevented.

The object of the present invention is to provide a residual stress improving method for a pipe imparting larger compressive residual stress on the pipe by applying the axial load under a predetermined condition before or at the same time the internal pressure of the pipe rises in a method of improving the residual stress by expanding the pipe by the internal pressure of the pipe.

SUMMARY OF THE INVENTION

The present invention is characterized to be a residual stress improving method for a pipe including the steps of: applying an axial load on a pipe with respect to a stress improving region having tensile residual stress in the pipe filled with fluid inside; raising the pressure inside the pipe; and performing plastic deformation on the stress improving region; in which the axial load on the pipe is in such a range of stress making axial strain of an outer surface of the pipe 0% or above and being yield stress of the pipe or below.

Also, the stress improving region is characterized to include a welding section of the pipe and a heat affected zone affected by the welding section, or a pipe expansion section of the pipe.

Also, pressure inside the pipe and the axial load of the pipe are characterized to be removed after the pipe is plastically deformed. Also, the pressure inside the pipe is characterized to be removed at a time point an outer surface of the pipe starts to be plastically deformed after an inner surface of the pipe starts to be plastically deformed. Also, pressure inside the pipe is characterized to be removed at a time point strain of the outer surface of the pipe reaches approximately 0.5%.

Also, plastic deformation is characterized to be confirmed by measuring variation of circumferential strain with respect to axial strain of the outer surface of the pipe.

Also, a detection position of the circumferential strain at the outer surface of the pipe is characterized to be in the vicinity of the welding section and a detection position of the axial strain is characterized to be in a region other than the vicinity of the welding section.

According to an aspect of the present invention, large compressive residual stress can be imparted to the inner surface of the stress improving region of the pipe, the residual stress condition can be improved, and stress corrosion cracking can be prevented by applying the axial load of stress with which the axial strain of the outer surface of the pipe is 0% or above and which is yield stress of the pipe or below in plastic deformation of the pipe when the internal pressure of the pipe is raised and the axial load is applied to the stress improving region where residual stress of the pipe is to be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic drawing showing a residual stress improving method of a pipe according to an aspect of the present invention.

FIG. 1B is a schematic drawing showing a residual stress improving method of a pipe according to an aspect of the present invention.

FIG. 1C is a schematic drawing showing a residual stress improving method of a pipe according to an aspect of the present invention.

FIG. 2 is an explanatory drawing for a structure of a pipe and the vicinity of a welding section of an object to be worked according to an aspect of the present invention.

FIG. 3 is a schematic drawing showing a loading device of an axial load on a pipe according to an aspect of the present invention.

FIG. 4 is a schematic drawing showing a pipe expansion method in the residual stress improving method of a pipe according to an aspect of the present invention.

FIG. 5A is a graph showing an axial strain condition optimum for residual stress improvement according to an aspect of the present invention.

FIG. 5B is a graph showing an axial strain condition optimum for residual stress improvement according to an aspect of the present invention.

FIG. 5C is a graph showing an axial stress condition optimum for residual stress improvement according to an aspect of the present invention.

FIG. 6 is a graph showing an axial stress condition optimum for residual stress improvement according to an aspect of the present invention.

FIG. 7A is a graph showing circumferential strain and stress when the internal pressure of a pipe rises according to an aspect of the present invention.

FIG. 7B is a graph showing axial and circumferential strain when the internal pressure of a pipe rises according to an aspect of the present invention.

FIG. 8 is an explanatory drawing showing an example evaluating the residual stress of the inner surface of a pipe of an object to be worked according to an aspect of the present invention.

FIG. 9A is a graph showing the circumferential residual stress of a pipe in which the weld residual stress exists according to an aspect of the present invention.

FIG. 9B is a graph showing the axial residual stress of a pipe in which the weld residual stress exists according to an aspect of the present invention.

FIG. 10 is an explanatory drawing showing an example evaluating the strain of the outer surface of a pipe of an object to be worked according to an aspect of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventors investigated a method capable of imparting larger compressive residual stress on the inner surface of the welding section and the vicinity of the welding section of a pipe. As a result, the present inventors have newly found out that it is preferable to impart large compressive residual stress on the inner surface of a stress improving region of the pipe where the residual stress of the pipe is to be improved by raising the axial load and the internal pressure of the pipe to perform plastic deformation with respect to the stress improving region and applying an axial load with which the axial strain of the outer surface of the pipe is 0% or above and the stress is the yield stress of the pipe or below in plastic deformation.

Thus, probability of occurrence of stress corrosion crack in a pipe can be further lowered by imparting larger compressive residual stress on the inner surface of the stress improving region of the pipe and reducing the tensile residual stress condition or changing the tensile residual stress condition to the compressive residual stress condition. Embodiments according to an aspect of the present invention reflecting the investigation result will be specifically described below.

[Basic Constitution of Residual Stress Improvement]

A residual stress improving method for a pipe of an appropriate embodiment according to the present invention will be described with an example of a case improving the residual stress in the vicinity of the welding section of a pipe in a power plant referring to FIGS. 1A-1C.

In FIG. 1A, a pipe 1 of a power plant made of a stainless steel (or nickel alloy) is joined by butt-weld at its end part. By the butt-weld, tensile residual stress causing stress corrosion cracking has occurred in the inner surface of a welding section 2 of the pipe 1 and a heat affected zone 3 where mechanical properties have changed due to the heat of welding existing adjacent to both sides of the welding section 2, and therefore the stress improvement is required. Accordingly, an axial tensile load 4 is applied as shown in FIG. 1B. Then, internal pressure 5 is applied as shown in FIG. 10, and the residual stress at the welding section 2 and the heat affected zone 3 is improved.

[Measurement of Residual Stress by Strain Gauge]

The position for measuring the strain occurring in the pipe 1 is to be decided. As shown in FIG. 2, in the vicinity of the welding section, the inner surface 11 of the pipe is subjected to groove preparation for matching the inner surfaces in welding and an groove preparation surface 13 is formed. In order to confirm improvement of tensile residual stress of the heat affected zone 3 and the groove preparation surface 13 of the inner surface 11 of the pipe, the measurement position of a strain gauge G1 in the circumferential direction of the pipe 1 is arranged in the vicinity of the welding section 2.

On the other hand, the axial strain greatly changes according to the measurement position because the groove preparation surface 13 and excess weld metal 14 exist in the vicinity of the welding section. Therefore, it is preferable to arrange the measurement position of a strain gage G2 in the axial direction of the pipe 1 in a position sufficiently departing from the welding section 2.

When the strain gauge G1 is adhered to an outer surface 12 of the pipe 1 inside the existing power plant in order to measure the circumferential strain, measurement of the magnitude of the internal pressure 5 is difficult. It is impossible to clearly determine whether the pipe 1 has plastically deformed only by variation of the circumferential strain. Accordingly, in such a case, the circumferential and axial strains are measured at a same measurement position to obtain the circumferential strain with respect to the axial strain, and thereby whether the pipe 1 has plastically deformed can be confirmed.

In order to measure the circumferential strain, the outside diameter of the pipe 1 may be measured instead of using a strain gauge. When the outside diameter of the pipe 1 is to be measured, measurement positions for the outside diameter decided are respectively marked on the outer surface 12 of the pipe 1 with an oil-based pen and the like to prevent that the measurement positions cannot be distinguished due to plastic deformation of the pipe 1 accompanying rise of the internal pressure 5. Measurement is performed with respect to the measurement positions of the outside diameter using the markings, and the measured values obtained are recorded as initial values of the measurement positions of the outside diameter.

In measuring them, two locations or more are measured within each cross-section perpendicular to the axis of the pipe 1 in the vicinity of the welding section 2 considering variation in the thickness of the pipe 1.

[Application of Axial Load]

A constitution for applying the axial load 4 on the pipe 1 is shown in FIG. 3. As a method for applying the axial load 4, hydraulic chucks 21 and hydraulic cylinders 22 are used. The hydraulic chucks 21 are attached on the upstream side and downstream side of the welding section 2 and the heat affected zone 3 where the residual stress of the pipe 1 is to be improved. In order to apply the axial load 4 between the hydraulic chucks 21, the hydraulic cylinders 22 are attached between the hydraulic chucks 21. In order to prevent occurrence of bending deformation in the pipe 1 when the axial load 4 is applied by the hydraulic cylinders 22, the hydraulic cylinders 22 are attached in two locations or more at a same distance in the circumferential direction of the pipe 1.

When the internal pressure 5 of the pipe 1 rises, a load by the internal pressure 5 also occurs in the axial direction of the pipe 1. Accordingly, when the internal pressure 5 rises, the axial load 4 applied initially by the hydraulic cylinders 22 also increases. The axial load 4 by the hydraulic cylinders 22 is to be controlled so that the axial stress of the pipe 1 does not exceed the yield stress of the pipe 1 when the internal pressure 5 of the pipe 1 rises.

[Method for Raising Internal Pressure]

The axial load 4 is applied to the pipe 1, and the internal pressure 5 of the pipe 1 is raised. A commonly known method for raising the internal pressure will be described referring to FIG. 4. As shown in (a) in FIG. 4, inner containers 32 and outer containers 33 are attached to the outer surface 12 of the pipe 1 on the upstream side and downstream side of the welding section 2 and the heat affected zone 3 where the residual stress of the pipe 1 is to be improved. Inside of the pipe 1 is filled with water 31.

Inside of each of the inner containers 32 and the outer containers 33 is hollow. As shown in (b), ethyl alcohol 34 is poured into the inner containers 32 and the outer containers 33 so as to immerse the pipe 1. Dry ice 35 is charged into the outer containers 33 filled with ethyl alcohol 34 through openings in the upper ends. The water 31 inside the pipe 1 is cooled at positions surrounded by the outer containers 33 by the dry ice 35 charged. Therefore, the water 31 is frozen at respective positions inside the pipe 1 to form ice plugs 36, and the water 31 is sealed by the pair of ice plugs 36.

In order to form the ice plugs 36 exerting firmer sealing performance inside the pipe 1, cooling of the pipe 1 by the dry ice inside the outer containers 33 is continued over the time longer than the time required for forming the ice plugs 36 obtained by an experiment and the like beforehand. Thus, the pair of ice plugs 36 firmly freezing to the inner surface of the pipe 1 are formed, and a sealed region filled with the water 31 is formed between the ice plugs 36.

Further, as shown in (c), the dry ice 35 is charged into the inner containers 32 filled with ethyl alcohol 34. The water 31 existing between the ice plugs 36 is cooled by the dry ice 35 inside the inner containers 32. In respective positions where the inner containers 32 are attached, the pipe 1 is cooled down to below the freezing point, and the water 31 existing between the ice plugs 36 starts to freeze. When the water 31 changes to ice 37, the volume expands, and therefore the pressure of the water 31 existing between the ice plugs 36 starts to rise. The pressure inside the pipe 1 rises between the ice plugs 36, and the pipe 1 forms a pipe expansion section 38 between the inner containers 32 outward in the radial direction by the pressure rise.

After the pipe 1 has been confirmed to be plastically deformed by deformation of the pipe 1 or the measured circumferential strain has reached a set value, the dry ice 35 in the inner containers 32 and the outer containers 33 is discharged to the outside. After all the ice plugs 35 and the ice 36 inside the pipe 1 have been molten and changed to the water 31 or at the same time of drop of the internal pressure of the pipe 1, the axial load 4 is removed. At this time, as shown in (d), the pipe 1 has been plastically deformed, and therefore is in a shape of expanded pipe even after the internal pressure 5 and the axial load 4 have been removed.

[Optimum Condition for Residual Stress Improvement]

A residual stress improving method for a pipe according to an aspect of the present invention will be described referring to FIGS. 5A-5C. With respect to the residual stress improving method for a pipe according to an aspect of the present invention, the present inventors analyzed the residual stress occurring in the pipe 1 by a finite element method and clarified the optimum condition for stress improvement.

The pipe 1 applied with the residual stress improving method for a pipe of an object of the present example was made stainless steel-make with 195,000 MPa Young's modulus, 0.3 Poisson's ratio, and 270 MPa yield stress. The inside diameter of the pipe 1 was 20 mm, and the thickness was changed in 3 steps of 2 mm, 6 mm and 10 mm.

The internal pressure 5 of the pipe 1 with 2 mm thickness shown in FIG. 5A was raised from the state of 0 MPa to 87 MPa. Also, with respect to the axial load occurring in the pipe 1 by the internal pressure 5 and the axial load 4 of the pipe 1, the load was applied to the end part of the pipe 1 as the axial distributed stress. The axial distributed stress of the pipe 1 with 2 mm thickness was made 129 MPa from the ratio of the inside diameter and outside diameter of the pipe 1 when there was no axial load 4, and was raised from the state of 0 MPa to 129 MPa simultaneously with the rise of the internal pressure 5 of the pipe 1. Here, the axial distributed stress means the distributed stress in the axial direction occurring in the pipe when a load is applied.

Next, with respect to the pipe 1 with 6 mm thickness shown in FIG. 5B, when there was no axial load 4, the internal pressure 5 was raised from 0 MPa to 147 MPa, and the axial distributed stress was raised from 0 MPa to 94 MPa simultaneously.

Further, with respect to the pipe 1 with 10 mm thickness shown in FIG. 5C, when there was no axial load 4, the internal pressure 5 was raised from 0 MPa to 216 MPa, and the axial distributed stress was raised from 0 MPa to 72 MPa simultaneously.

With respect to the pipe 1 with each thickness, the axial stress calculated from the cross-sectional area of the pipe 1 when the axial load 4 was applied was applied as the axial distributed stress. For example, when the axial load 4 of 49 kN is applied to the pipe 1 with 6 mm thickness, the axial stress of 100 MPa occurs in calculation from the cross-sectional area of the pipe 1. Accordingly, adding 100 MPa to the axial stress of 147 MPa occurring by the internal pressure 5 of the pipe 1, 247 MPa is applied as the axial distributed stress.

FIGS. 5A-5C show the result of evaluation of the optimum condition for stress improvement by the finite element method when the axial load is changed. The axis of abscissa of FIGS. 5A-5C represents the axial strain occurring in the outer surface 12 of the pipe 1 when the axial load 4 and the internal pressure 5 of the pipe 1 reach the maximum values described above. Also, the axis of ordinates represents the residual stress occurring in the inner surface 11 of the pipe 1 after the internal pressure 5 and the axial distributed stress of the pipe 1 have been removed, and as it goes to the minus region, the compressive residual stress becomes higher. Zero in the axis of ordinates represents the state of a raw pipe without any load and without any internal pressure.

From FIGS. 5A-5C, it is obvious that the improving effect of the axial residual stress is enhanced when the axial strain exceeds 0%. Accordingly, it is preferable to make the axial strain of the outer surface 12 of the pipe 1 when the internal pressure 5 and the axial load 4 are applied on the pipe 1 0% or above. Also, the improving effect of the circumferential residual stress becomes highest when the axial strain of the outer surface 12 of the pipe 1 is approximately 0%.

FIG. 6 shows the upper limit of the improving effect of the axial stress at the inner surface 11 of the pipe 1 with respect to the axial distributed stress applied to the end part of the pipe 1. From FIG. 6, it is known that the improving effect drops, or the improving effect drops than in the state without any axial load 4, when the axial distributed stress exceeds the yield stress of 270 MPa. From these results, the axial load 4 with which the axial strain at the outer surface 12 of the pipe 1 is 0% or above and the axial stress of the pipe 1 does not exceed the yield stress is preferable.

[Relation Between Circumferential Stress and Strain]

FIGS. 7A, 7B show the result of evaluation of the variation when the internal pressure 5 of the pipe 1 with 6 mm thickness without the axial load 4 is raised by the finite element method.

FIG. 7A shows variation of the circumferential stress at the inner surface 11 and the outer surface 12 with respect to the circumferential strain at the outer surface 12 when the internal pressure 5 of the pipe 1 is raised. When the internal pressure 5 is low and the circumferential strain is small, the pipe 1 is elastically deformed, and therefore the circumferential stresses occurring at the inner surface 11 and the outer surface 12 of the pipe 1 are generally equal.

When the internal pressure 5 continues to rise, the pipe 1 starts to be plastically deformed. At this time, in the radial direction of the pipe 1, the inner surface 11 is applied with the internal pressure 5, whereas the outer surface 12 is in the state of without load. Therefore, the circumferential stress at which plastic deformation starts differs between the inner surface 11 and the outer surface 12 of the pipe 1, and difference in stress occurs between the inner surface 11 and the outer surface 12 of the pipe 1.

When the internal pressure 5 of the pipe 1 is removed, the pipe 1 shrinks in the radial direction by the amount of elastic deformation, and the circumferential stress drops. Because the drop amount of the circumferential stress is generally equal between the inner surface 11 and the outer surface 12 of the pipe 1, as the stress difference between the inner surface 11 and the outer surface 12 occurring after the pipe 1 is plastically deformed is larger, the improving effect of the residual stress becomes higher.

From FIG. 7A, it is known that the improving effect becomes highest when the outer surface 12 of the pipe 1 starts to be plastically deformed which is the time when the circumferential strain at the outer surface 12 is approximately 0.5%. Here, by measuring variation of the circumferential strain with respect to the axial strain in the same location of the outer surface 12 of the pipe 1, whether the outer surface 12 of the pipe 1 has been plastically deformed is determined.

FIG. 7B shows variation of the circumferential strain with respect to the axial strain at the outer surface 12 when the internal pressure 5 of the pipe 1 is raised. When the circumferential strain is approximately 0.5% at which the improving effect of the residual stress becomes highest, variation amount of the circumferential strain with respect to the axial strain greatly varies.

When the pipe 1 has been deformed beforehand by welding and the like, the circumferential strain at which the pipe 1 starts to be plastically deformed differs from that in the present example. Therefore, plastic deformation of the pipe 1 is determined by measuring the variation of the circumferential strain with respect to the axial strain at the outer surface 12 of the pipe 1.

[Improvement Result of Residual Stress]

The residual stress improvement result of the pipe according to an aspect of the present invention will be described referring to FIG. 8. The present inventors evaluated the residual stress improving effect in the present embodiment by the finite element method on the case the pipe 1 has non-uniform shape in the axial direction by the welding section 2 and the weld residual stress exists.

The pipe 1 applied with the residual stress improving method for a pipe of an object of the present embodiment was made stainless steel-make with 195,000 MPa Young's modulus, 0.3 Poisson's ratio, and 270 MPa yield stress. The inside diameter of the pipe 1 at the location other than the welding section 2 and the groove preparation surface 13 was 25 mm, and the thickness was 4.5 mm.

FIG. 8 shows the residual stress at the inner surface 11 of the pipe 1 evaluated by the finite element method. It is known that the tensile residual stress of 100 MPa-300 MPa has occurred at the welding section 2 and the groove preparation surface 13.

FIGS. 9A, 9B show examples of the residual stress distribution at the inner surface 11 after the internal pressure 5 and the axial load 4 of the pipe 1 improved by the present embodiment have been removed, evaluated by the finite element method. When the axial load was not applied, the internal pressure 5 of the pipe 1 was raised from 0 MPa to 110 MPa and the axial distributed stress was raised from 0 MPa to 130 MPa simultaneously. In the regard, the axial distributed stress was changed as 100 MPa, 130 MPa (axial load was not applied), 200 MPa, and 300 MPa.

The axial strain of the outer surface 12 of the pipe 1 at the position of 20 mm from the center of welding when the internal pressure 5 of the pipe 1 had reached 110 MPa was −0.04% when the axial distributed stress was 100 MPa, was 0.02% when the axial distributed stress was 130 MPa, was 0.18% when the axial distributed stress was 200 MPa, and was 1.23% when the axial distributed stress was 300 MPa.

From FIG. 9A, it is known that all of the residual stresses in the circumferential direction of the pipe in the welding section are positioned largely in the minus region representing the compressive residual stress and show improvement of the stress condition. Also, at 300 MPa at which the axial distributed stress exceeds the yield stress, the circumferential stress improving effect is low in the vicinity of the groove preparation surface.

Further, from FIG. 9B, it is known that all of the residual stresses in the axial direction of the pipe in the welding section are positioned in the minus region representing the compressive residual stress and show improvement of the stress condition. Also, at 100 MPa of the axial distributed stress at which the axial strain becomes 0% or below when the internal pressure 5 is applied, the axial stress becomes 0 MPa or above in a part in the vicinity of the groove preparation surface which shows the tensile stress condition.

Accordingly, even in a pipe in which deformation and residual stress exist beforehand by welding, the axial load 4 with which the axial strain at the outer surface 12 of the pipe 1 is 0% or above and the axial stress of the pipe 1 does not exceed the yield stress is preferable.

FIG. 10 shows the result of analysis of the circumferential and axial strains at the outer surface 12 of the pipe 1 by the infinite element method when the internal pressure 5 and the axial load 4 are applied on the pipe 1 improved by the present embodiment. FIG. 10 shows that the deformation amount of the part other than the heat affected zone 3 of the pipe 1 becomes larger than the deformation amount of the heat affected zone 3 according to the shape of the welding section 2 when the pipe 1 is expanded. Accordingly, it is preferable to arrange the measurement position of the strain gauge G1 in the circumferential direction of the pipe 1 in the vicinity of the welding section 2 in order to confirm that the residual tensile stresses of the heat affected zone 3 and the groove preparation surface 13 of the inner surface 11 of the pipe have been improved.

Also, the axial strain greatly changes in the vicinity of the welding section by the influence of the groove preparation surface 13 and the excess weld metal 14. Therefore, it is preferable to arrange the measurement position of the strain gauge G2 in the axial direction of the pipe 1 in a position sufficiently departing from the welding section 2. 

1. A residual stress improving method for a pipe comprising the steps of: applying an axial load on a pipe with respect to a stress improving region having tensile residual stress in the pipe filled with fluid inside; raising the pressure inside the pipe; and performing plastic deformation on the stress improving region, wherein the axial load on the pipe is in such a range of stress making axial strain at an outer surface of the pipe 0% or above and being yield stress of the pipe or below.
 2. The residual stress improving method for a pipe according to claim 1, wherein the stress improving region includes a welding section of the pipe and a heat affected zone affected by the welding section.
 3. The residual stress improving method for a pipe according to claim 1, wherein the stress improving region includes a pipe expansion section of the pipe.
 4. The residual stress improving method for a pipe according to claim 1, wherein pressure inside the pipe and the axial load of the pipe are removed after the pipe is plastically deformed.
 5. The residual stress improving method for a pipe according to claim 4, wherein the pressure inside the pipe is removed at a time point when an outer surface of the pipe starts to be plastically deformed after an inner surface of the pipe starts to be plastically deformed.
 6. The residual stress improving method for a pipe according to claim 4, wherein pressure inside the pipe is removed at a time point when circumferential strain of the outer surface of the pipe reaches approximately 0.5%.
 7. The residual stress improving method for a pipe according to claim 5, wherein plastic deformation is confirmed by measuring variation of circumferential strain with respect to axial strain at the outer surface of the pipe.
 8. The residual stress improving method for a pipe according to claim 7, wherein a detection position of the circumferential strain at the outer surface of the pipe is in the vicinity of the welding section and a detection position of the axial strain is in a region other than the vicinity of the welding section. 